Lithium ion batteries, also known as Li-ion Batteries or LIB's are widely used in consumer electronics, for example in mobile telephones, tablets and laptops. LIB's are also used in other fields, such as military uses, electric vehicles and aerospace applications. During discharge of the battery, lithium ions (Li ions) travel from a high-energy anode material through an electrolyte and a separator to a low-energy cathode material. During charging, energy is used to transfer the Li ions back to the high-energy anode assembly. The charge and discharge processes in batteries are slow processes, and can degrade the chemical compounds inside the battery over time. Rapid charging causes accelerated degradation of the battery constituents, as well as a potential fire hazard due to a localized, over-potential build-up and increased heat generation—which can ignite the internal components, and lead to explosion.
Typical Li-ion Battery anodes contain mostly graphite. Silicon, as an anode-alloying component, generally exhibits higher lithium absorption capacities in comparison to anodes containing only graphite. Such silicon-containing electrodes, however, usually exhibit poor life cycle and poor Coulombic efficiency due to the mechanical expansion of silicon upon alloying with lithium, and upon lithium extraction from the alloy, which reduce the silicon alloy volume. Such mechanical instability results in the material breaking into fragments.
There are several types of materials, which are widely used as the anode material. Carbon in its graphite form, to date, is still the anode material of choice in lithium ion batteries. The electrochemical activity of carbon comes from the intercalation of lithium ions between the graphene layers, where one lithium ion is occupying approximately 6 atoms of carbon to give theoretical specific capacity of ˜372 mAh/g and volumetric capacity of 330-430 mAh/cm3, which is limited by the layer structure of the graphite. Moreover, due to the intercalation mechanism of lithium ions in graphite, the charging and discharging rates are yet limited, and hence bound to metallization of lithium, mainly during fast charging followed by slow discharging.
However, graphite anode material is cheap and has a limited expansion (e.g., shrinkage during lithium intercalation). single crystalline graphitic particles undergo uniaxial 10% strain along the edge planes, in addition, the Solid-Electrolyte Interface (SEI) formation is stable, and can be formed also on the intercalation basal planes and thus the SEI also preferentially forms on these planes as well. Nevertheless, the SEI can still be damaged from this expansion-shrinkage mechanism, being one of the reasons for capacity fading.
Unlike graphite, lithium titanium oxide (LTO) has a successful commercial usage due to the combination of superior thermal stability and high discharging and charging rate capability. However, LTO has low gravimetric capacity and higher cost due to the titanium. The high charging/discharging rates capability is due to a more stable microstructure, i.e. “zero strain” intercalation mechanism in combination with a high potential lithiation. The latter is also the reason for a low cell potential however, the volume change of LTO during lithiation—delithiation is less than 0.2%. Nevertheless, surface reactions are not avoidable using LTO anodes. Such anodes suffer from severe gassing due to reaction between the organic electrolyte and the LTO active material. To overcome this issue, there are several reports suggesting the use of carbon coating. However, carbon (and also graphite) may catalyze and accelerate electrolyte decomposition in the formation of the SEI, especially at high temperatures and/or high surface potential.
The use of silicon (Si) which serves as a conversion material, namely, as anode active material, has on one hand much higher gravimetric and volumetric capacity for lithium, but on the other hand suffers from high volumetric changes during charging/discharging cycles which may cause low cyclability.
However, the use of Silicon (Si) as anode active material may have three major problems: (I) large volume changes during charge and discharge cycles; (II) instability of the Solid-Electrolyte Interface (SEI) also due to the volume changes; and (III) fracturing during discharge of the silicon, which may be caused both by the volume change on one hand and the kinetics of the metal-metal bonding and disbonding reaction on the other.